Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

[Mihai Carabas]

La 22.11.2023 22:51, Christoph Lameter a scris:
> On Mon, 20 Nov 2023, Mihai Carabas wrote:
>
> > cpu_relax on ARM64 does a simple "yield". Thus we replace it with
> > smp_cond_load_relaxed which basically does a "wfe".
>
> Well it clears events first (which requires the first WFE) and then 
> does a WFE waiting for any events if no events were pending.
>
> WFE does not cause a VMEXIT? Or does the inner loop of 
> smp_cond_load_relaxed now do 2x VMEXITS?
>
> KVM ARM64 code seems to indicate that WFE causes a VMEXIT. See 
> kvm_handle_wfx().

In KVM ARM64 the WFE traping is dynamic: it is enabled only if there are 
more tasks waiting on the same core (e.g. on an oversubscribed system).

In arch/arm64/kvm/arm.c:

 457 >-------if (single_task_running())
 458 >------->-------vcpu_clear_wfx_traps(vcpu);
 459 >-------else
 460 >------->-------vcpu_set_wfx_traps(vcpu);

This of course can be improved by having a knob where you can completly 
disable wfx traping by your needs, but I left this as another subject to 
tackle.